For fishfinders that is one type of underwater detection devices, increasing in detection distance is demanded to detect a shoal of fish at a deeper location. In order to increase the detection distance, generally, a transmission pulse length is simply lengthened. However, if the transmission pulse length is lengthened, an echo reflected from a target object will also be longer, and the axial resolving power will be reduced. Thus, a method of increasing the axial resolving power may be adopted in which a transmission pulse is frequency-modulated, and a received echo signal and a replica waveform of the transmitted signal is correlation-processed to perform a pulse compression of the received signal. An underwater detection device that performs such a pulse compression process is disclosed in JP2005-249398 (A).
However, in the underwater detection device that performs the pulse compression process, as shown in FIG. 10, false images referred to as “range side lobes” appear at positions before and after a main lobe, that shows a position of a detection target object, of a pulse-compressed signal.
When the range side lobes appear, especially due to a seabed, the false images may be mistakenly viewed by an operator as a target object, such as a school of small fish. In addition, an image of a shoal of fish located near the seabed may be difficult to view and a seabed depth may mistakenly be determined.